Any matter of fact that a party to a lawsuit offers to prove or disprove an issue in the case. A system of rules and standards that is used to determine which facts may be admitted, and to what extent a judge or jury may consider those facts, as proof of a particular issue in a lawsuit.
Until 1975, the law of evidence was largely a creature of the COMMON LAW: Evidence rules in most jurisdictions were established by cases rather than by organized, official codifications. Legal scholars long pushed for legislation to provide uniformity and predictability to the evidentiary issues that arise during litigation. Following a lengthy campaign begun by the American Law Institute, which drafted its Model RULES OF EVIDENCE in 1942, and the National Conference of Commissioners on Uniform State Rules, which drafted the Uniform Rules of Evidence in 1953, Congress in 1975 adopted the FEDERAL RULES OF EVIDENCE. The Federal Rules of Evidence are the official rules in federal court proceedings. Most states now also have codified rules of evidence based on these federal rules. Both state and federal rules of evidence serve as a guide for judges and attorneys so that they can determine whether to admit evidence?that is, whether to allow evidence to be observed by the judge or jury making factual conclusions in a trial.
One important benchmark of admissibility is relevance. Federal Rule of Evidence 402 states, in part, "All relevant evidence is admissible, except as otherwise provided." The goal of this rule is to allow parties to present all of the evidence that bears on the issue to be decided, and to keep out all evidence that is immaterial or that lacks PROBATIVE value. Evidence that is offered to help prove something that is not at issue is immaterial. For example, the fact that a defendant attends church every week is immaterial, and thus irrelevant, to a charge of running a red light. Probative value is a tendency to make the existence of any material fact more or less probable. For instance, evidence that a murder defendant ate spaghetti on the day of the murder would normally be irrelevant because people who eat spaghetti are not more or less likely to commit murder, as compared with other people. However, if spaghetti sauce were found at the murder scene, the fact that the defendant ate spaghetti that day would have probative value and thus would be relevant evidence.
The most common form of evidence is the testimony of witnesses. A witness can be a person who actually viewed the crime or other event at issue, or a witness can be a person with other relevant information?someone who heard a dog bark near the time of a murder, or who saw an allegedly injured plaintiff lifting weights the day after his accident, or who shared an office with the defendant and can describe her character and personality. Any competent person may testify as a witness, provided that the testimony meets other requirements, such as relevancy.
The Federal Rules of Evidence contain broad competency requirements. To testify, a witness must swear or affirm that he or she will testify truthfully; possess personal knowledge of the subject matter of the testimony; have the physical and mental capacity to perceive accurately, record, and recollect fact impressions; and possess the capacity to understand questions and to communicate understandably, with an interpreter if necessary. When an issue of state law is being determined, the state rules of evidence govern the competency of a witness. States that have not adopted the Federal Rules of Evidence may have other grounds for INCOMPETENCY, such as mental incapacity, immaturity, religious beliefs, and criminal convictions. The Federal Rules of Evidence and most jurisdictions state that jurors and presiding judges are not competent to testify in the case before them.
To be admissible, testimony must be limited to matters of which the witness has personal knowledge, meaning matters that the witness learned about using any of his or her senses. Second, the witness must declare under oath or affirmation that the testimony will be truthful. The purpose of this requirement is to "awaken the witness' conscience and impress the witness' mind with the duty to [be truthful]" (Fed. R. Evid. 603). The oath or affirmation requirement also serves as a ground for perjury if the witness does not testify truthfully. Although the oath frequently invokes the name of God, the witness need not possess any religious beliefs; a secular affirmation is sufficient.
Witnesses may be called to testify by any party to the lawsuit. The party who calls a witness to testify generally questions the witness first, in what is known as direct examination. The judge may exercise reasonable control over the questioning of witnesses in order "(1) to make the interrogation and presentation effective for the ascertainment of the truth; (2) to avoid needless consumption of time, and (3) to protect the witnesses from harassment, or undue embarrassment" (Fed. R. Evid. 611(a)). Thus, the judge may prevent a witness from rambling in a narrative fashion and may require an attorney to ask specific questions in order to ascertain the truth quickly and effectively.
The federal rules and most jurisdictions discourage the use of leading questions on direct examination. These are questions that are designed to elicit a particular answer by suggesting it. For example, the question "Didn't the defendant then aim the gun at the police officer?" is a leading question, and normally it would not be permitted on direct examination. By contrast, "What did the defendant do next?" is a nonleading question that would be permitted on direct examination. In most cases, questions that can be answered with either "Yes" or "No" are considered to be leading questions. Courts generally will permit leading questions during direct examination if the witness is adverse or hostile toward the questioning party.
Leading questions are permitted, and are common practice, during cross-examination. Once a party conducts a direct examination, the opposing party is entitled to cross-examine the same witness. The scope of questions asked during cross-examination is limited to the subject matter that was covered during direct examination, and any issues concerning the witness's credibility. Attorneys use cross-examination for many purposes, including eliciting from a witness favorable facts; having the witness modify, explain, or qualify unfavorable versions of disputed facts elicited during direct examination; and impeaching, or discrediting, the witness.
If a witness is a lay witness (i.e., not testifying as an expert), the witness generally may testify as to facts and not as to opinions or inferences, unless the opinions or inferences are "(a) rationally based on the perception of the witness and (b) helpful to a clear understanding of the witness' testimony or the determination of a fact in issue" (Fed. R. Evid.). For example, a witness may not testify that she smelled marijuana unless she can sufficiently establish that she knows what marijuana smells like. Lay witnesses commonly testify about such things as the speed that a car was going, or someone's approximate age, but these types of inferences are less likely to be permitted the more closely they address critical issues in the case.
"If scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue, a witness who is qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise" (Fed. R. Evid. 702). The admissibility of EXPERT TESTIMONY hinges on whether such testimony would help the judge or jury, and whether the witness is properly qualified as an expert. Expert witnesses may, and usually do, testify in the form of an opinion. The opinion must be supported by an adequate foundation of relevant facts, data, or opinions, rather than by conjecture. Thus, an expert frequently relies on firsthand or secondhand observations of facts, data, or opinions perceived prior to trial, or presented at trial during testimony or during a hypothetical question posed by an attorney. Courts do not require experts to have firsthand knowledge of facts, data, or opinions because experts in the field do not always rely on such firsthand knowledge. For instance, physicians routinely make diagnoses based on information from several sources, such as hospital records, X-ray reports, and opinions from other physicians.
When an expert offers a scientific fact as substantive evidence or as the basis of his or her opinion, the court must determine the reliability of the scientific fact by looking at such things as the validity of the underlying scientific principle, the validity of the technique applying that principle, adherence to proper procedures, the condition of instruments used in the process, and the qualifications of those who perform the test and interpret the results. Issues frequently arise over such scientific tools and techniques as lie detectors, DNA testing, and hypnosis. Some scientific tests, such as drug tests, radar, and PATERNITY blood tests, generally are accepted as reliable, and their admissibility may be provided for by statute.